Game Suggestion ....
When a player joins a new team, have an automatic drop in game shape.
This would be to simulate the 'getting acquainted' period of the new player and his new team. It would have the effect of making a manager either try to work a new player into his squad through scrimmage games, or to take a chance with tossing him straight into the first squad.
So, while his physical condition didn't suddenly drop when he changed clubs, it seems to make sense to have a period of a few games where the new player's performance constantly improves as he gets to know his new teammates and the systems they play .... and having the Game Shape drop and then recover seems to model that rather nicely.